2750
Anal. Chern. 1986, 58, 2750-2756
pF cm-2) and presumably wide range of microscopic surface area. ACKNOWLEDGMENT We thank Daniel Fagan for carrying out the ESCA measurements. We also thank T. Kuwana, I.-F. Hu, R. Mark Wightman, and Royce Engstrom for useful discussions and for providing preprints of their papers on carbon activation. Registry No. C, 7440-440;Fe(CN)63-,13408-62-3;Fe(CN):-, 13408-63-4; NaDH, 58-68-4; 02, 7782-44-7; DOPAC, 102-32-9; DHBA, 37491-68-2; ascorbic acid, 50-81-7;catechol, 120-80-9; hydroquinone, 123-31-9;dopamine, 51-61-6. LITERATURE CITED Adams, R. N. Electrochemistry at Solid Nectrodes ; Marcel Dekker: New York, 1969. Bishop, E.; Htchcock, P. H. Analyst (London) 1973, 98, 475. Kinoshlta, K. I n Mcdern Aspects of Ek&ochemIsty; Bockris, J. OW., Conway, 8. E., White, R. E., Eds.; Plenum: New York, 1982; p 557, and references therein. Amatore, C.; Saveant, J. M.; Tessier, D. J. Nectroanal. Chem. 1983. 746, 37. Goldstein, E. L.; Van de Mark, M. R. Nectrochim. Acta 1982, 27, 1079. S. Gilman Lectroanal. 1967, 2 , 111. Conway, 8. E.; et al. Anal. Chem. 1973, 45, 1331. Rusiing, J. F. Anal. Chem. 1984, 5 6 , 578. Kamau, G. N.; Willis, W. S.; Rusling, J. F. Anal. Chem. 1985, 5 7 , 545. Thomton, D. C.; Corby, K. T.; Spendel, V. A,: Jordan, J.; Robbat, A,; Rutstrom, D. J.: Gross, M.; Ritzler, G. Anal. Chem. 1985, 5 7 , 150. Laser, D.; Ariel, M. J. Nectroanal. Chem. 1974, 5 2 , 291. Gunsingham, H.; Fleet, 9. Analyst (London) 1982, 107, 896. Hu, I. F.; Karweik, D. H.; Kuwana, T. J. Nectroanal. Chem. 1985, 788, 59. Plock, C. E. J. Electroanal. Chem. 1969, 2 2 , 185. Taylor, R. J.; Humffray, A. A. J. Electroanal. Chem. 1973, 4 2 , 347. Engstrom, R. C. Anal. Chem. 1982, 5 4 , 2310. Engstrom, R. C.; Strasser. V. A. Anal. Chem. 1884, 5 6 , 136. Blaedel, W. J.; Jenkins, R. A. Anal. Chem. 1974, 4 6 , 1952.
(19) Molroux, J.; Elving, P. J. Anal. Chem. 1978. 50, 1056. (20) Wightman, R. M.; Palk, E. C.; Borman, S.; Dayton, M. A. Anal. Chem. 1978, 50, 1410. (21) Cabaniss, G. E.; Diarnantis, A. A.; Murphy, W. R., Jr.; Linton, R. W.; Meyer, T. J. J. Am. Chem. Soc. 1985, 107, 1845. (22) Wang, J.; Hutchins, L. D. Anal. Chim. Acta 1985, 767, 325. (23) Wang, J. Anal. Chem. 1981, 53, 2280. (24) Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1961, 5 3 , 1386. (25) Rice, M. E.; Gaius, Z.; Adams, R. N. J. Electroanal. Chem. 1983, 743, 89. (26) Falat, L.; Cheng, H. Y. J. Necfroanal. Chem. 1983, 157, 393. (27) Stutts, K. J.; Kovach, P. M.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1883, 55, 1832. (28) Fagan, D. T.; Hu, I. F.; Kuwana, T. Anal. Chem. 1985, 5 7 , 2759. (29) Miller, C. W.; Karwelk, D. H.; Kuwana, T. Anal. C h m . 1981, 5 3 , 2319. (30) Evans, J.; Kuwana, T. Anal. Chem. 1979, 57, 358. (31) Hershenhart, E.; McCreery, R. L.; Knight, R. D. Anal. Chem. 1984, 5 6 , 2257. (32) Nicholson, R. S.Anal. Chem. 1965,3 7 , 1351. (33) Kawaik, J.; Jedral, T.; Galus, 2 . J. Electroanal. Chem. 1883, 745, 183. (34) Kazee, 8.;Weisshaar, D. E.; Kuwana, T. Anal. Chem. 1985, 5 7 , 2736. (35) Murray, R. W. Nectroanal. Chem. 1884, 13. (36) Panzer, R. E.; Elving, P. J. Necfrochlm. Acta 1975, 2 0 , 635. (37) Biurton, K. F. Nectrochh. Acta 1973, 18, 869. (38) Daum, P. H.; Enke, C. G. Anal. Chem. 1989, 47, 653. (39) Harden, E. D.; Fan. T. P.; Blakley, C. R.; Vestal, M. L. Anal. Chem. 1984, 5 6 , 2. (40) Duley, W. W. Laser Processing and Analysls of Materials; Plenum: New York, 1983; p 69. (41) Ready, J. F. Effects of High Power Laser Radiation; Academic Press: New York, 1971. (42) Ulrlch, R. K.: Alkire, R. C. J. Nectrochem. Soc. 1981, 728, 1169. (43) Kuiken, H. K.: Mlkkers, F. E. P.; Wierenger, P. E. J . Electrochem. Soc. 1883, 730, 554.
RECEIVED for review March 26,1986. Accepted July 1, 1986. This work was funded by the OSU Materials Research Laboratory and by the NSF Division of Chemical Analysis.
Amperometric Response of Microlithographically Fabricated Microelectrode Array Flow Sensors in a Thin-Layer Channel Lawrence E. Fosdick and James L. Anderson*
Department of Chemistry, T h e University of Georgia, Athens, Georgia 30602 Thomas A. Baginski and Richard C. Jaeger
Alabama Microelectronics Science and Technology Center, Department of Electrical Engineering, Auburn University, Auburn University, Alabama 36849-3501 Mlcroelectrode arrays were fabrlcated by use of photollthograpMc techniques. The electrodes consisted of gold metal deposlted on a silicon substrate, pretreated by thermal oxldation to form a thln surface layer of silicon dloxlde. These electrodes were tested In a rectangular flow cell under laminar flow conditions. The experlmental steady-state response of these electrodeis was evaluated by uslng flow lnJectlonmthodokgy and compared wHh theoretlcal behavlor predicted by backward lmpllclt flnlte difference slmulatlons. Currents and their dependence on flow rate show good agreement wlth theory. Aglng tests of the electrodes indlcate that the response decreased over a period of a few hours, but the electrodes remalned actlve and showed no physical degradation after 24 h of continuous use.
Microelectrode arrays have received a great deal of attention as sensors for amperometric flow detectors in liquid chromatography and flow injection analysis (1-10). The theoretical
response of microelectrode arrays in flow cells has been treated by a number of workers ( 1 , 2 , 11-14). The theory has been developed to characterize microelectrode arrays that have regularly spaced and sized active elements in the flow stream, for two fundamental geometries. The first geometry consists of a series of identical rectangular electrode elements maintained at a common applied potential and separated by rectangular gaps or electrochemically inactive sites ( I ) . The second geometry consists of rectangular active elements separated by narrow gaps, where alternating elements are maintained a t different potentials, also referred to as an interdigitated microelectrode array (11). Related work with rectangular strip electrodes has treated a single pair of electrodes in series at different potentials in a flow stream (121, an array of rectangular electrodes in a thin-layer cell in the absence of flow (15),an array in static solution (16),and polymer-coated arrays (17-19). The microelectrode array flow sensors reported in the literature have included irregular arrays based on graphite powder (4-7,9, IO), reticulated vitreous carbon ( 3 ) ,orderly
0003-2700/86/0358-2750$01.50/00 1968 American Chemical Society
JALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
(8) and disorderly (20) arrays of carbon fiber tips, and an orderly array of gold used in a membrane-covered oxygen sensor (21). The theory presently deals with orderly microelectrode arrays, though some studies have also theoretically characterized the effects of irregular arrays (1)or progressively varying geometry (13). Experimental evaluation of the developed theory for regular arrays is necessary to assess properly the benefits of microelectrode array flow sensors. The present paper reports on the experimental results obtained for a series of regular microelectrode arrays produced by using photolithographic techniques. Other workers have also recently reported microlithographic fabrication of microelectrode arrays (15-19,21). The primary goals of this work are evaluation of fabrication methodology, comparison of experimental results with theoretical predictions, and assessment of the robustness of these electrodes in flow streams. Additionally, a modification of the equation developed by Filinovsky (2) to calculate the relative response of a microelectrode array compared to a solid electrode of equal geometric area has been developed which provides much closer agreement with the digital simulations than previously reported (1).
THEORY The steady-state, diffusion-controlled current response for a microelectrode array on one wall of a thin-layer channel can be calculated by solution of the equation
.OOOEtOO
n
a
i
U
CL
,000E-01
l
t
:
1.000E-01
:
3.000E-01
5.000E-01
e
V , = [6U/(bWc)l(z/b)(l - z / b ) (2) Here, U is the volume flow rate, W, is the channel width, and z is the height at which the velocity is calculated.
Solutions of eq 1 under laminar flow conditions have been reported in the literature for a solid electrode (22, 23), a microelectrode array in which all active elements are maintained at a common potential (1,2,14),and an interdigitated set of electrodes in which alternate elelpents are maintained at different potentials (11). Response at array electrodes has been numerically simulated using the backward implicit finite difference (BIFD) method ( I , 11,23). In all cases, longitudinal and lateral diffusion have been neglected. Filinovsky derived an approximate expression based on a linearization of eq 2 to predict the relative current response, p(N,0),of a microelectrode array in which all elements are maintained at a single potential relative to a solid electrode of equal geometric area ( 2 ) ,according to
where N is the number of active sites, and 0 is the fraction of the area that is electrochemically inactive (2). This result was obtained by taking the harmonic mean of the first terms of series solutions for two limiting cases for which limiting calculations were possible, namely, the case 0 0 and the case 0 1. The assumption of a linear velocity profile restricts the validity of this approximation to situations involving very thin diffusion layers, e.g., for high flow rates.
-
-
7.000E-01
9.000E-01
Figure 1. Comparison of response of microelectrode arrays from eq 5 (asterisk), to BIFD predictions (solid curves) and Filinovsky's predictions (broken curves), for (a) N = 10 and (b) N = 100.
An earlier paper reported a discrepancy between the BIFD digital simulations and the predictions of Filinovsky ( I ) , resulting from use of the harmonic mean rather than from any error in the limiting cases treated. An empirical modification of eq 3 has been developed, based on the first two terms of Filinovsky's limiting expression for low active area (eq 4) which agrees well with both BIFD simulations and experimental results over a wide range of conditions. According to Filinovsky = W [ ( i- e)/e12/3(i - &[(I
where D is the diffusion coefficient of the electroactive species, C is the concentration, z is the direction of diffusion perpendicular to flow, x is the direction of flow, and V, is the flow velocity parallel to the electrode. Equation 1 is valid for a thin rectangular flow channel of height b parallel to the z axis, under conditions of fuUy developed laminar flow. Longitudinal and lateral diffusion are neglected. The flow velocity under laminar flow conditions can be calculated from the equation
2751
- 0 ) / 0 1 ~ / 3 ] (4)
where parameter a was not explicitly determined, while parameter k was set equal to 1/3 (2). This limiting equation was substituted in eq 3, and a least-squares analysis of parameters a and k was carried out, using a sequential simplex algorithm written in FORTRAN-77 on a DEC Pro380 microcomputer, by fitting the modified equation to a number of theoretical data sets generated using the BIFD simulation for several values of N. The resulting equation is
P ( ~ , 8 =) {W3[(i - ~ ) / e ] ~ /-~a2vk[(i ( i - e ) / 0 ] ~ 1 x~ ] [I - 0 . 4 s ~ / ~ / ~ / ~ ] } / { N -1 /0)/8]2/3{1 ~[(1 - alvk[(i6)/8]2/3] + 1 - 0.404/3/N1/3](5) where a and k are the parameters to be fit by the simplex. Data generated by the BIFD simulation for values of N ranging from 5 to 100 were used to estimate the parameters. Each BIFD data set contained 22 points, for 0 values of 0 to 1. The values for a and k that gave the best simplex fit for all data sets were -2.5577 and -0.030 362 9, respectively. The response predicted by eq 5 is in good agreement with that from the BIFD simulation. The largest relative deviation between eq 5 and the BIFD simulation occurs for a microelectrode array of three elements, which was outside the range of N used in the simplex, with 0 = 0.85, at 2.4%. Typical deviations for microelectrode arrays of five elements or more are less than 1.5%. The error is not constant but varies slightly, in a periodic manner, as 0 varies from 0 to 1. Plots are shown in Figure 1of the responses predicted by eq 5 using the values for a and It determined by the simplex, by the BIFD simulation, and by Filinovsky's harmonic approximation (eq 3). A plot of the sum of the squares of the residuals as a function of N shows a weak dependence on the number of electrodes but asymptotically approaches a value of ca. 1.9 X 10" as the number of electrodes becomes large, as shown in Figure 2. The negative value of parameter a in eq 5 is counter to intuition, since a positive value of a in Filinovsky's limiting expression (eq 4) represented an expected decrease in response for active sites downstream of the first electrode, due to shielding. Similarly, the small value for the pafameter k is not intuitively obvious. These discrepancies are believed to
2752
I I
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
o PO
40
8'0
80
Number E l e c t r o d e s
Flgure 2. Sum of squares of differences between efficiencies predicted by BIFD calculations and by eq 5, as a function of the number of active sites. Efficiencies are calculated for microelectrode arrays relative to solid electrodes of equal geometric area.
arise principally from the use of the harmonic approximation, which does not adequately represent the transition between limiting cases. Thus these added empirical parameters are practically useful but do not provide much theoretical insight. Since Equation 5 calculates the response of a microelectrode array relative to a solid electrode of equal geometric area, combination with the calculation for a solid electrode under the same conditions can be used to predict the response of a microelectrode array. An equation reported by Weber and Purdy (22) to calculate the current at a solid electrode in a rectangular flow cell is Isoli,j = nFUCo(W e / W,)[ 1.40326L2- 0.80246L3] (6) where n is the number of electrons, F is the Faraday constant, C" is the initial concentration of the electroactive species, We is the electrode width, W , is the channel width, and bL is the thickness of the diffusion layer at the trailing edge of the electrode of total length L , calculated from the relation 0.93556L3 - 0.60186L4= LD W,/ Ub (7) Equation 7 requires an iterative solution for bL. Equation 6 shows excellent agreement with the BIFD simulation for a solid electrode in a rectangular flow channel under steady-state conditions (23). Combining eq 5 and 6, and using eq 7 to calculate the value of bL, provides a useful equation to calculate the current response at a microelectrode array:
where L becomes the total geometric length of the microelectrode array, and all other terms retain their previous designations. This equation provides a reasonable estimate of the current at a microelectrode array, eliminating the need to perform the complete BIFD calculation. T h e calculation time for a BIFD simulation is dependent on the number of active sites, requiring ca. 1.1s per active site or gap on a VAX 11/750 computer (a 100-element microelectrode array requires 219 s of CPU time), while the time required for eq 8 is significantly shorter and independent of N . EXPERIMENTAL SECTION The microelectrode arrays used in this study were fabricated by using conventional photolithographic and metalization techniques on silicon wafers, 2 in. in diameter and 325 gm thick. Gold conductor was deposited on a photoresist-treated silicon substrate that had been pretreated by thermal oxidation in dry oxygen to produce a 400 nm thick insulating layer of silicon dioxide. Electrode Fabrication.
Flgure 3. Photograph of microelectrode array mask. Design allows use as two different electrode geometries. The first geometry uses only half of the total active sites (attached to a single bus bar), while the second uses all active sites.
All operations involving the application and development of the photoresist were performed in a clean room under yellow light, to avoid undesirable reactions of the resist. The silicon wafers were spin coated with an image reversal photoresist, consisting of 100 parts Shipley 13505to 1part Monalzoline (Mona Industries, Inc.). The Monalzoline adds an imidazole moiety to a normally positive resist (24). The coating was performed by pipetting 1 mL of the resist onto a 2 in. diameter silicon wafer, followed by rotation a t 3800 rpm for 24 s to produce a uniform coating on the wafer. Thickness measurements, performed with a Dektak thickness monitor, indicated an average resist thickness of 1100 nm. The resist film was then dried for 10 min at 100 "C. The photographic masks used for selective exposure of the coated wafers were produced by software written in BASIC on an IBM personal computer. The data files created by this program were of the form used to control a Gyrex Pattern Generator, Model 1005, through an IBM computer, to expose the glass photographic mask plates in the patterns desired for the microelectrode arrays. Each plate defined two different patterns, each in duplicate. A typical microelectrode array is shown in Figure 3. The coated wafers were exposed to ultraviolet light through the electrode masks for 24 s using a Kasper mask aligner. This operation resulted in cross-linkingthe resist coating in areas where the light contacted the resist by converting the base-insoluble photosensitive component of the resist into a carboxylic acid, which combined with the imidazole to form a salt. The wafers were then baked for 30 min at 100 "C, decomposing the carboxylic acid salt moiety in the exposed areas of the resist film, releasing carbon dioxide, and rendering the exposed film insoluble to the alkaline developer. The wafers were then exposed for 24 s in the mask aligner without a mask to force reaction of the photosensitive compound and imidazole in the previously unexposed areas, converting it to a base-soluble carboxylic acid-imidazole salt. During the developing step, the resist originally masked by the electrode design was removed. Thus, photoresist was present on the silicon wafer in all areas where gold would be absent in the final electrode array. After developing, the wafers were treated in an oxygen plasma to remove any residual material from the development process. The gold was deposited onto the wafers in a Varian electron beam evaporation system to a measured thickness of 220 nm. The coated wafers were then soaked in acetone, which dissolved the photoresist, lifting the gold in areas where photoresist was present at the time of the evaporation process. The final electrodes were then rinsed in methanol followed by water, and dried with filtered air. The electrodes on each wafer were separated by scoring and breaking. Flow I n j e c t i o n E x p e r i m e n t s . Flow injection experiments were used to study the response characteristics of the microelectrode arrays. The flow system consisted of a Varian 8500 syringe pump to minimize pump noise and a pneumatically actuated Valco injector. An injection volume of 800 gL, coupled with a 90-wL dead volume, ensured that a steady-state concentration equal to the injected concentration was achieved at the injection peaks for flow rates up to 4.0 mL/min (21). This was verified by the use of a 1.1-mL injection volume for some experiments. An amperometric detector (Bioanalytical Systems,
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Model LC-3A) served to control potential and monitor current. The potentiostat was modified to have a 47-ms time constant on the current-to-voltage converter, to minimize distortion of the steady-state peak shape and height. The mobile phase contained 0.1 F, pH 7.1 phosphate buffer in distilled/deionizedwater. The analytes used in this study were ferrocenecarboxylicacid (FCA) (Aldrich, used as received) and 1,l’-bis(hydroxymethy1)ferrocene(BHMF) (synthesized locally (6)).Stock solutions of accurately known analyte concentrations of ca. 1 mM were prepared fresh daily in the phosphate buffer and then accurately diluted to ca. 0.25 pM to 1.0 MMfor the flow injection experiments. The microelectrode array, mounted on a screw-driven platform, was compressed against a 51 pm thick spacer (Tefzel ZOO),which served as a solution flow channel placed between the microelectrode array and the opposite wall of stainless steel. The flow stream entered and exited the cell through orifices in the opposite wall which also served as the auxiliary electrode. A reference electrode (Ag/AgC1/1.0 M KCl) was mounted in the exit stream. The flow channel was 0.3 cm wide and 0.8 cm long, resulting in a total cell volume of 1.2 pL. Some experiments were performed using a double-sided adhesive tape (3M) as the spacer material. These experiments required a completely aqueous mobile phase to minimize the dissolution of the adhesive. Cyclic voltammograms and diffusion coefficient determinations were performed using an EG&G Princeton Applied Research Corp. Model 174.4 potentiostat combined with a Model 175 wave form programmer. The working electrode was a 0.3 cm diameter glassy carbon electrode mounted in a locally constructed rotator based on a Moto-Matic Model E-550 motor-tachometer generator. RESULTS AND DISCUSSION Electrode Fabrication. The yield from the electrode fabrication process was less than 100%. The liftoff process used is designed for a thick coating of photoresist, which becomes undercut during the initial developing step (24). This undercutting serves to weaken severely any connections between the gold deposited over the resist and the gold deposited in resist-free regions. Thus, when the resist is dissolved in acetone during the final step, there are few gold metal-metal bonds to be broken. For the proper degree of undercutting, a resist f i i thickness of 5 to 7 pm is recommended. The resist film thickness in these experiments was only 1.1pm, resulting in some electrodes becoming damaged when the liftoff step was performed, due to the tensile strength of the gold. Future plans include the use of a thicker resist layer to be achieved by decreasing the amount of solvent in the resist or by coating multiple resist layers on the wafers before exposure through the photographic masks. Due to the weak adhesion of the gold on the oxidized silicon substrate and the thin layer of gold present (220 nm), no electrode pretreatment was performed. After fabrication, the electrodes were stored in plastic containers to prevent contamination by airborne matter. Cyclic voltammograms performed on the microelectrode arrays and a solid gold electrode pretreated with conventional polishing techniques showed similar background response in the phosphate buffer, with peaks presumably due to oxidative formation and reductive stripping of a gold oxide layer at potentials of +970 and +485 mV vs. Ag/AgCl, respectively. The relatively weak adhesion of gold on silicon dioxide was expected to be a potentially serious problem in a flow cell, where the combined effects of various electrode reactions, including oxide formation, and shear forces due to solution flow, might result in erosion or lifting of the gold from the substrate. These phenomena were not observed on any of the electrodes tested. Applied potentials were kept less positive than +0.5 V vs. Ag/AgCl, except for brief periods, to avoid interference by gold oxide formation (25, 26). Oxides have the effect of creating an additional microelectrode arraylike structure due to partial blockage of the active electrode
2753
1.000EW0
4
w
\ H
0.000E-01
Flgure 4. Hydrodynamic voltammogram of FCA in 0.1 M phosphate
buffer, pH 7.1 (asterisk), normalized to the mass transport-limited current, and the response calculated from eq 9 using the determined values of k, and Eo’(solid curve).
surface, eventually blocking the electrode completely. Three microelectrode arrays of different geometries were used in the flow cell for at least 24 h. Inspection under 50X magnification indicated no physical differences after overnight usage with an applied potential of +450 mV vs. Ag/AgCl, when compared to unused microelectrode arrays. Potentials ranging from -500 mV to +900 mV were applied for short times (less than 10 min). These extreme potentials did not visibly damage the electrodes during these brief periods. An electrochemical gold pretreatment described by Oesch and Janata results in etching of up to 50 nm of gold from the electrode surface, which could damage the thin (220nm) gold layer used in this experiment, and so was avoided (25). Electrode Response. Steady-state theories derived from eq 1 for the current response of electrodes, both solid (12,22, 23, 27, 28) and microelectrode arrays ( I , 2, 11, 14), assume steady-state response of the electrode under laminar flow conditions. Steady-state response can be attained, near the center of a flow injection peak, if the injection volumes are sufficiently large relative to the dead volume between injector and detector. The injection volume required to achieve steady-state conditions in the cell can be calculated for given values of system dead volume and flow rate (29). The electrochemical cell design and injection volume were selected to ensure these conditions experimentally. Theoretical calculations assume a known active area for the microelectrode array. Oxide film formation or electrode fouling, where foreign substances adsorb onto the electrode surface, may change the effective active electrode area. To minimize this effect, experiments were performed at short (ca. 1-2 h) times after the initial application of the working potential. Theoretical current response calculations were performed by use of the BIFD method described previously ( I , 23) and eq 8. The diffusion coefficient of FCA was determined both by chronoamperometry at a stationary glassy carbon electrode and by rotating disk voltammetry at a glassy carbon electrode using the Levich equation and had an average value of (7.1 f 0.6) X cm2/s, where the uncertainty is the standard deviation between the two measurements. Hydrodynamic voltammograms were obtained to determine the potential range where the current is dependent only on concentration and flow rate and independent of the applied potential. A typical hydrodynamic voltammogram of FCA in phosphate buffer is shown in Figure 4. The most negative potential at which mass transport-limited current was attained was selected (+450 mV vs Ag/AgCl) to minimize the rate of oxide formation and maximize the useful life of the electrodes. Plots of E as a function of log [(IL - 4/11,where E is the applied potential, ILis the mass-transport-limited current,
2754
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13,NOVEMBER 1986
Table I. Heterogeneous Rate Constants Determined for FCA no. of electrodes
8
Eom
k,
1 12 12 16
0.0 0.74 0.90 0.74
311 315 318 325
0.0155 0.0289 0.0209 0.0293
- 8) 0.0155 0.111 0.209 0.113
2.600Et02
d
..
"
C H
.Eo',mV vs. Ag/AgCl (1.0 M KC1). and Z is the measured current at potential E, were linear, with slopes close to the 59.2-mV value expected for a reversible system (30),indicating that the oxidation of FCA is quasireversible in the phosphate buffer. Data from hydrodynamic voltammograms of FCA obtained with several microelectrode arrays were analyzed to determine the electron transfer rate constant (31),defined by
6.000€+01
+
5.000E+01
1.500Ei.02
Flow Rate
2.500€+02
(ml/hr)
Figure 5. Experimental response (asterisk) of microelectrode arrays compared to theoretical predictions (solid curves), for FCA in phosphate buffer: (a) N = 16, 0 = 0.74,C o = 1.43 x IO-^ M; (b) N = 12,o = 0.74,C o = 2.23 X lo-' M.
where
mR = 1.467(V/[WJ])1/3(D/b)2/3 (10) Here, It is the mass-transport-limited current, Z is the current at potential E , mR and mo are the mass transport coefficients to the electrode for the reduced and oxidized forms, respectively, E"' is the formal reduction potential, a is the electron transfer coefficient, R is the gas constant, and T is the temperature in K. The other terms were defined previously. For simplification, nowas assumed to be equal to mR and a was assumed to be 0.5. A simplex algorithm, written in FORTRAN-77 on a DEC Pro380 computer, was used to solve eq 9 for k , and E"'. The results are summarized in Table 1. The heterogeneous rate constant for the FCA redox reaction was sufficiently fast under experimental conditions that the hydrodynamic voltammograms deviated only slightly from the reversible limits, so that the small measurement uncertainties yielded relatively large uncertainties in k,. It is interesting to note that the apparent rate constant before normalization for active area was larger for the arrays than for the solid electrode. This observation suggests that the decrease in active area associated with a microelectrode array does not necessarily increase susceptibility to poisoning. Insufficient data are available for reliable quantitative evaluation of the dependence of apparent k, on fractional active area. The limiting current response of the microelectrode arrays is in excellent agreement with the theoretical predictions, as shown in Table 11. To ensure reliable data, only measurements performed within 1h of the initial potential application
z t
0.000E-01 1.000E-01
3.000E-01
5.000E-01
e
7.000E-01
9.OOOE-01
Figure 8. Experimental response (points) of microelectrode arrays, normalized to the response of a s o l i electrode of equal geometric area compared to response curves based on eq 8 (solid curves) and Filinovsky's predictions (eq 3): (A)N = 1, 8 = 0 (solid electrode); ( * ) N = 32, 0 = 0.50;(0) N = 16,0 = 0.74;(+) N = 12,0 = 0.74;(0) N = 8,0 = 0.73;( X ) N = 24, 0 = 0.80;(0) N = 12, 0 = 0.90.
were used to test the theory. The experimental response of a microelectrode array as a function of flow rate is compared to the theoretical response in Figure 5 . This agreement is typical when the flow dependence experiments are performed shortly after initial installation of the microelectrode array in the cell to minimize the effects caused by electrode fouling. The theory for amperometric detector response predicts that the measured mass-transport-limited, steady-state current should be proportional to the cube root of flow rate for both solid electrodes and microelectrode arrays ( I , 2 , 2 2 , 2 3 , 2 7 ) , as long as the diffusion layer is much thinner than the flow channel. Experimentally observed flow dependences of several
Table 11. Experimental Results and Theoretical Predictions" no. of elements 1
8 8 12 12 16 24 32 32
8
C, fiM
spacer thickness, cm
exptl
0.00 0.73 0.73 0.74 0.90 0.74 0.80 0.50 0.50
0.243 0.368 1.34 0.558 0.267 1.43 1.73 1.43 1.64
0.0051 0.0051 0.0076 0.0051 0.0051 0.0051 0.0076 0.0051 0.0076
28.7 f 0.4 26.4 f 0.5 55.2 f 0.4 41.9 f 0.5 12.6 f 0.3 115 f 2 106 f 0.8 145 f 4 134 f 0.8
I , nA BIFD
eq 8
flow rate exponent
29.6 27.0 59.5 42.6 12.9 114 101 153 134
29.5 27.2 59.6 42.9 12.9 115 102 152 134
0.31 0.33 0.32 0.31 0.33 0.33
"Current response of microelectrode arrays compared to predictions for the BIFD method and eq 8 a t a flow rate of 1.00 mL/min, a t ambient temperature. Channel width = 0.30 cm, channel height and initial concentration of FCA are noted in table. Uncertainties represent one standard deviation.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
4.500EIO2
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Time ( m i n ) Figure 7. Background current density for microelectrode arrays as a function of time after inltial potenthi application: (a) N = 12, 8 = 0.897, active area = 0.0146 cm2;(b) N = 8, 8 = 0.731, active area = 0.0384 cm2;(c) N = 1, 8 = 0 (solid),active area = 0.15 om2. microelectrode arrays, extracted from plots of log i vs. log U, agree with this predicted flow dependence, as shown in Table 11. Comparison of experimental vs. theoretical current efficiencies as a function of fractional blockage is illustrated in Figure 6. It is clear that experimental results are in good agreement with BIFD predictions for the entire range of N and 8 values examined. It is also apparent that the slight protrusion (220 nm) of the gold electrode elements above the silicon dioxide surface of the microelectrode array does not significantly disrupt the laminar flow profde or otherwise cause deviations from predictions for a perfectly planar microelectrode array under the conditions of this study. After a few hours of use, the flow rate exponent decreased slightly, and current reached a stable response, approximately 25% below the initial or theoretical response after overnight operation. Fresh electrodes yielded slopes for log current vs. log flow rate of 0.32-0.33. After overnight use, this slope decreased to 0.27429. This result is similar to that reported by Weber and co-workers, who attributed their observations to the formation of a film on the electrode surface which restricted the diffusion of the analyte to the electrode surface (28). It is postulated that an oxide film slowly forms on the electrode surface, allowing restricted diffusion to the electrode surface but preventing bulk flow of the solution to the surface. Oesch and Janata have shown that several monolayers of gold oxides suffice to block electron transfer (25,261. The measured background current density indicates the presence of some surface reaction. The background currents decreased with time, were directly proportional to the initial active area of the microelectrode arrays, and were independent of the flow rate. A plot of background current density as a function of time is shown in Figure 7 for a solid electrode and two microelectrode arrays with different active areas. The background current continued to decrease slowly during overnight experiments. It is clear that background current densities per unit active area converge to essentially the same limit a t long times, regardless of the fraction of the geometric area of the electrode that is blocked. The experiments using the double-sided adhesive tape as the spacer material showed a more marked decrease of mass-transport-limited current response with time than observed for the Tefzel spacer, with approximately 25% loss of response after 2 h. This drastic change in response is believed to be due to migration of the cyanoacrylate adhesive over the electrode surface, forming an insulating layer over the electrodes. Thus, care is required in applying adhesive-backed spacers in electrochemical thin-layer cells, despite their appeal for minimizing leakage. A major advantage of microelectrode arrays over solid electrodes of similar geometric area is an increased signal/noise
3.OOOEtOl
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Time ( m i n ) Flguro 8. Current response of 1.0 I.~MFCA normalized to background current or noise as a function of time after initial potential application. Electrodes a, b, and c are described in Figure 7. ratio ( I , 3, 4, 7-10). This increase in signal/noise results l area, while the signal because noise is dependent on t ~ t aactive is dependent on the geometric area of the electrode. The noise was usually below the measurement capabilities of the instrumentation, where current measurements of less than 1nA could be performed with no noticeable noise. However, the background current of a solid electrode also interferes with the current measurement. A large background current coupled with a small analyte current can result in measurement errors, since a difference current is needed. Measurement of the background current density as a function of time indicates that the background current is indeed directly proportional to the initial active area of the electrode (Figure 7). Figure 8 shows the ratio of signal to background current for the electrodes studied, indicating that the sparser electrodes provide lower peak currents, but better signal-to-background response, compared to solid electrodes of the same geometric area. CONCLUSIONS The experimental results obtained for a series of microelectrode arrays show good agreement with the theoretical calculations, indicating that the fundamental model developed is a useful tool in the development and optimization of microelectrode array flow sensors. Further experiments are required, using a wider range of fractional active areas, to provide a more complete test of the differences between the theoretical predictions obtained by our group and those of Filinovsky (2), which disagree on the expected response when the fraction of inactive area is small (23). The results obtained to date, however, are in encouraging agreement with our model. These results indicate that microelectrode arrays show promise for decreasing detection limits for electrochemical detectors in flow systems. Though the gold electrodes are too fragile for routine use, they are sufficiently robust to verify the utility of the theory and serve to justify further development of more robust microelectrode arrays and electrochemical flow cells. The results obtained here also encourage the extension of microelectrode array applications, including the use of interdigitated microelectrode arrays, which show promise of further enhancements of signal/noise ratio and selectivity (11). Microelectrode arrays show great promise for enhancing the applicability and versatility of electrochemical detectors in flow cells. Registry No. Gold, 7440-57-5; silicon, 7440-21-3. LITERATURE CITED ( 1 ) Moldoveanu, S.; Anderson, J. L. J . Elecboanal. Chem. 1985, 785, 239-252. (2) Flllnovsky, V. Yu. Electrochim. Acta 1980, 25, 309-314.
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Anal. Chem. 1986, 58, 2756-2761 Sleszynski, N.; Osteryoung J.; Carter, M. Anal. Chem. 1984, 56, 130- 135. Anderson, J. L.: Chesney, D. J. Anal. Chem. 1980, 52,2156-2161. Anderson, J. E.; Tallman. D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978, 50, 1051-1056. Chesney, D. J.; Anderson, J. L.; Weisshaar, D. E.; Tallman, D. E. Anal. Chim. Acta 1981, 124, 321-331. Weisshaar, D. E.; Tallman, D. E.;Anderson, J. L. Anal. Chem. 1981, 53, 1809-1813. Caudill, W. L.; Howell, J. 0.; Wightman, R. M. Anal. Chem. 1982, 54, 2532-2536. Tallman, D. E.; Weisshaar. D. E. J . Lip. Chromatogr. 1983, 6, 2157-2172. - . Anderson. J. L.;Whiten, K. K.; Brewster, J. D.; Ou, T. Y.; Nonidez, W. K. Anal. Chem. 1985, 57, 1366-1373. Anderson, J. L.; Ou, T. Y.; Moldoveanu, S. J , Electroanal. Chem. 1985. 196, 213-226. Aoki, K.; Tokuda, K.; Matsuda, H. J. Nectroanal. Chem. 1977, 79, 49-78. Fosdick. L. E.; Anderson, J. L. Anal. Chem., 1988, 58,2481-2465. Cope, D. K.; Tallman, D. E. J. Electroanal. Chem., in press. Anderson, L. B.; Sanderson, J. L. Anal. Chem. 1985, 57, 2388-2393. Thormann, W.; van den Bosch, P.; Bond, A. M. Anal. Chem. 1985, 57, 2764-2770. Kittlesen, G. P.; White, H. S.;Wrighton, M. S. J. Am. Chem. SOC. 1984, 106. 7389-7396. White, H. S.;Kittlesen, G. P.; Wrighton, M. S. J . Am. Chem. SOC. 1984, 106, 5375-5377.
(19) Chidsey, C. E.; Feldman. B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1988, 58,601-607. (20) Belal, F.; Anderson, J. L. Analyst (London) 1985, 110, 1493-1496. (21) Siu, W.; Butler J.; CobboM R. S.C. Procwdlngs of the International Conference on Biomedical Transducers, 1975: Vol. 1, pp 319-324. (22) Weber, S.G.;Purdy, W. C. Anal. Chim. Acta 1978, 100, 531-544. (23) Anderson, J. L.; Moldoveanu, S. J. Electroanal. Chem. 1984. 179, 107- 1 17. (24) Moritz, H. I€€€ Trans. Electron Devices 1985, ED-32,672-676. (25) Oesch, U.;Janata, J. Electrochlm. Acta 1983, 28, 1237-1246. (26) Oesch, U.; Janata, J. Electrochlm. Acta 1983, 28, 1247-1253. (27) Weber, S.G. J. Electroanal. Chem. 1983, 145, 1-7. (28) Elbicki, J. M.; Morgan, D. M.; Weber, S. G. Anal. Chem. 1984, 56, 978-985. (29) Meschi, P. L.; Johnson, D. C. Anal. Chim. Acta 1981, 124,303-314. (30) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; Chapter 1. (31) Anderson, J. L. International Symposium on LCEC and Voltammetry, Indianapolis, IN, May 1983, Abstract No. 1.
RECEIVED for review April 28,1986. Accepted July 11, 1986. This work was supported by the U.S. Department of the Interior, Office of Water Research and Technology, EG&G Princeton Applied Research Corporation, and the Alabama Research Institute.
Electrodeposition and Characterization of Platinum Microparticles in Poly(4-vinylpyridine) Film Electrodes Duane E. Bartak,*' Beth Kazee, Katsuaki Shimazu, and Theodore Kuwana2 Department of Chemistry, T h e Ohio State University, Columbus, Ohio 43210
Electrodeposltlon of Pt micropartlcles at microgram levels In three types of M y ( 4-vlnylpyrldine) (PVP) films on glassy carbon (gc) electrodes Is described. The PVP films were formed by (1) electrochemical polymerization, (2) splncoaUng linear PVP on the gc surface, and (3) crob&linWng the llnear PVP on the gc surface. SEM photomicrographs of these flhns revealed the general structure, morphology, and degree of adhesion of the polymer to the carbon surface. Electrochemlcal polymerlzatlon of 4-vInylpyrldkre was carrled out In dlmethylformamlde by potentlostatic techniques on an anodically pretreated gc surface. Cross-llnklng of PVP was accomplished by heatlng a mixture of llnear polymer, triallyl-substituted cross-linking agent,and radkal bdtlator on the gc surface. Electrochemical reduction of an acidic solution of hexachloroplathate,which was allowed to penetrate the PVP films, produced three-dlmenslonal dispersion of Pt micropatticks. The Pt/PVP/gc electrodes exhiblted good actlvity with regard to the generation of hydrogen. The staMllty of the cross-linked PVP films In acid solution was conskJeraMy better than the linear PVP films.
An important application of polymer-modified electrodes is their utilization in electrocatalysis (1). Wrighton has utilized an electroactive, viologen-based polymer into which Pt or Pd was dispersed to improve hydrogen evolution on semiconductor electrodes (2, 3). In addition, it has been shown that Pd deposition in the viologen-based polymer results in high Present address: Department of Chemistry, University of N o r t h Dakota, G r a n d Forks, ND 58202. *Present address: Center of Bioanalytical Research, 2099 Constant Ave., University of Kansas, Lawrence. K S 66046.
current efficiencies for the reduction of bicarbonate to formic acid ( 4 ) . More recently, Wrighton demonstrated that Rh or Pd deposition in a cobaltocenium redox polymer on a p-type photocathode resulted in improved hydrogen generation (5). Our laboratory recently reported on the electrochemical deposition of metal microparticles into poly(viny1acetic acid) (PVAA) f i i s on glassy carbon electrodes (6,7). In particular, platinum was shown to be effectively dispersed in the film in a manner that resulted in significant catalytic activity with regard to the electrochemical generation of hydrogen. Furthermore, the stability of the platinum microparticles which were dispersed in the film was reported to be considerably better than microparticles deposited on a "baren glassy carbon (gc) surface (6). The PVAA film on the gc surface was produced by refluxing neat vinylacetic acid at 165 "C under nitrogen for a minimum of 16 h. However, the thickness of the PVAA f i i on the gc surface was difficult to control during the reflux and monolayer coverages were often the result of this process (8). This report describes the preparation and utilization of alternative polymer films whose thickness and properties can be better controlled for three-dimensional metal microparticle deposition. The three-dimensionality of metal microparticles in these films is important so as to attain high surface areas without agglomerization of the catalytic metal which should result in improved activities. In addition, it will be shown that cross-linking polymers directly on the gc surface can result in a film with a higher degree of stability. Poly(viny1pyridine) (PVP)has been shown to be an effective matrix for the incorporation of a variety of transition-metal ions into polymer electrodes (9). In particular, Anson and co-workers have shown that protonated PVP films can effectively bind transition-metal complexes by both covalent bonding and electrostatic interactions. Due to our interest
0003-2700/S6/0356-2756$01.50/0 1986 American Chemical Society
